1<?xml version="1.0" encoding="UTF-8"?>
2<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
3 "http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd" []>
4 5<book id="lk-hacking-guide">
6 <bookinfo>
7 <title>Unreliable Guide To Hacking The Linux Kernel</title>
8 9 <authorgroup>
10 <author>
11 <firstname>Rusty</firstname>
12 <surname>Russell</surname>
13 <affiliation>
14 <address>
15 <email>rusty@rustcorp.com.au</email>
16 </address>
17 </affiliation>
18 </author>
19 </authorgroup>
20 21 <copyright>
22 <year>2005</year>
23 <holder>Rusty Russell</holder>
24 </copyright>
25 26 <legalnotice>
27 <para>
28 This documentation is free software; you can redistribute
29 it and/or modify it under the terms of the GNU General Public
30 License as published by the Free Software Foundation; either
31 version 2 of the License, or (at your option) any later
32 version.
33 </para>
34 35 <para>
36 This program is distributed in the hope that it will be
37 useful, but WITHOUT ANY WARRANTY; without even the implied
38 warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
39 See the GNU General Public License for more details.
40 </para>
41 42 <para>
43 You should have received a copy of the GNU General Public
44 License along with this program; if not, write to the Free
45 Software Foundation, Inc., 59 Temple Place, Suite 330, Boston,
46 MA 02111-1307 USA
47 </para>
48 49 <para>
50 For more details see the file COPYING in the source
51 distribution of Linux.
52 </para>
53 </legalnotice>
54 55 <releaseinfo>
56 This is the first release of this document as part of the kernel tarball.
57 </releaseinfo>
58 59 </bookinfo>
60 61 <toc></toc>
62 63 <chapter id="introduction">
64 <title>Introduction</title>
65 <para>
66 Welcome, gentle reader, to Rusty's Remarkably Unreliable Guide to Linux
67 Kernel Hacking. This document describes the common routines and
68 general requirements for kernel code: its goal is to serve as a
69 primer for Linux kernel development for experienced C
70 programmers. I avoid implementation details: that's what the
71 code is for, and I ignore whole tracts of useful routines.
72 </para>
73 <para>
74 Before you read this, please understand that I never wanted to
75 write this document, being grossly under-qualified, but I always
76 wanted to read it, and this was the only way. I hope it will
77 grow into a compendium of best practice, common starting points
78 and random information.
79 </para>
80 </chapter>
81 82 <chapter id="basic-players">
83 <title>The Players</title>
84 85 <para>
86 At any time each of the CPUs in a system can be:
87 </para>
88 89 <itemizedlist>
90 <listitem>
91 <para>
92 not associated with any process, serving a hardware interrupt;
93 </para>
94 </listitem>
95 96 <listitem>
97 <para>
98 not associated with any process, serving a softirq or tasklet;
99 </para>
100 </listitem>
101 102 <listitem>
103 <para>
104 running in kernel space, associated with a process (user context);
105 </para>
106 </listitem>
107 108 <listitem>
109 <para>
110 running a process in user space.
111 </para>
112 </listitem>
113 </itemizedlist>
114 115 <para>
116 There is an ordering between these. The bottom two can preempt
117 each other, but above that is a strict hierarchy: each can only be
118 preempted by the ones above it. For example, while a softirq is
119 running on a CPU, no other softirq will preempt it, but a hardware
120 interrupt can. However, any other CPUs in the system execute
121 independently.
122 </para>
123 124 <para>
125 We'll see a number of ways that the user context can block
126 interrupts, to become truly non-preemptable.
127 </para>
128 129 <sect1 id="basics-usercontext">
130 <title>User Context</title>
131 132 <para>
133 User context is when you are coming in from a system call or other
134 trap: like userspace, you can be preempted by more important tasks
135 and by interrupts. You can sleep, by calling
136 <function>schedule()</function>.
137 </para>
138 139 <note>
140 <para>
141 You are always in user context on module load and unload,
142 and on operations on the block device layer.
143 </para>
144 </note>
145 146 <para>
147 In user context, the <varname>current</varname> pointer (indicating
148 the task we are currently executing) is valid, and
149 <function>in_interrupt()</function>
150 (<filename>include/linux/interrupt.h</filename>) is <returnvalue>false
151 </returnvalue>.
152 </para>
153 154 <caution>
155 <para>
156 Beware that if you have preemption or softirqs disabled
157 (see below), <function>in_interrupt()</function> will return a
158 false positive.
159 </para>
160 </caution>
161 </sect1>
162 163 <sect1 id="basics-hardirqs">
164 <title>Hardware Interrupts (Hard IRQs)</title>
165 166 <para>
167 Timer ticks, <hardware>network cards</hardware> and
168 <hardware>keyboard</hardware> are examples of real
169 hardware which produce interrupts at any time. The kernel runs
170 interrupt handlers, which services the hardware. The kernel
171 guarantees that this handler is never re-entered: if the same
172 interrupt arrives, it is queued (or dropped). Because it
173 disables interrupts, this handler has to be fast: frequently it
174 simply acknowledges the interrupt, marks a 'software interrupt'
175 for execution and exits.
176 </para>
177 178 <para>
179 You can tell you are in a hardware interrupt, because
180 <function>in_irq()</function> returns <returnvalue>true</returnvalue>.
181 </para>
182 <caution>
183 <para>
184 Beware that this will return a false positive if interrupts are disabled
185 (see below).
186 </para>
187 </caution>
188 </sect1>
189 190 <sect1 id="basics-softirqs">
191 <title>Software Interrupt Context: Softirqs and Tasklets</title>
192 193 <para>
194 Whenever a system call is about to return to userspace, or a
195 hardware interrupt handler exits, any 'software interrupts'
196 which are marked pending (usually by hardware interrupts) are
197 run (<filename>kernel/softirq.c</filename>).
198 </para>
199 200 <para>
201 Much of the real interrupt handling work is done here. Early in
202 the transition to <acronym>SMP</acronym>, there were only 'bottom
203 halves' (BHs), which didn't take advantage of multiple CPUs. Shortly
204 after we switched from wind-up computers made of match-sticks and snot,
205 we abandoned this limitation and switched to 'softirqs'.
206 </para>
207 208 <para>
209 <filename class="headerfile">include/linux/interrupt.h</filename> lists the
210 different softirqs. A very important softirq is the
211 timer softirq (<filename
212 class="headerfile">include/linux/timer.h</filename>): you can
213 register to have it call functions for you in a given length of
214 time.
215 </para>
216 217 <para>
218 Softirqs are often a pain to deal with, since the same softirq
219 will run simultaneously on more than one CPU. For this reason,
220 tasklets (<filename
221 class="headerfile">include/linux/interrupt.h</filename>) are more
222 often used: they are dynamically-registrable (meaning you can have
223 as many as you want), and they also guarantee that any tasklet
224 will only run on one CPU at any time, although different tasklets
225 can run simultaneously.
226 </para>
227 <caution>
228 <para>
229 The name 'tasklet' is misleading: they have nothing to do with 'tasks',
230 and probably more to do with some bad vodka Alexey Kuznetsov had at the
231 time.
232 </para>
233 </caution>
234 235 <para>
236 You can tell you are in a softirq (or tasklet)
237 using the <function>in_softirq()</function> macro
238 (<filename class="headerfile">include/linux/interrupt.h</filename>).
239 </para>
240 <caution>
241 <para>
242 Beware that this will return a false positive if a bh lock (see below)
243 is held.
244 </para>
245 </caution>
246 </sect1>
247 </chapter>
248 249 <chapter id="basic-rules">
250 <title>Some Basic Rules</title>
251 252 <variablelist>
253 <varlistentry>
254 <term>No memory protection</term>
255 <listitem>
256 <para>
257 If you corrupt memory, whether in user context or
258 interrupt context, the whole machine will crash. Are you
259 sure you can't do what you want in userspace?
260 </para>
261 </listitem>
262 </varlistentry>
263 264 <varlistentry>
265 <term>No floating point or <acronym>MMX</acronym></term>
266 <listitem>
267 <para>
268 The <acronym>FPU</acronym> context is not saved; even in user
269 context the <acronym>FPU</acronym> state probably won't
270 correspond with the current process: you would mess with some
271 user process' <acronym>FPU</acronym> state. If you really want
272 to do this, you would have to explicitly save/restore the full
273 <acronym>FPU</acronym> state (and avoid context switches). It
274 is generally a bad idea; use fixed point arithmetic first.
275 </para>
276 </listitem>
277 </varlistentry>
278 279 <varlistentry>
280 <term>A rigid stack limit</term>
281 <listitem>
282 <para>
283 Depending on configuration options the kernel stack is about 3K to 6K for most 32-bit architectures: it's
284 about 14K on most 64-bit archs, and often shared with interrupts
285 so you can't use it all. Avoid deep recursion and huge local
286 arrays on the stack (allocate them dynamically instead).
287 </para>
288 </listitem>
289 </varlistentry>
290 291 <varlistentry>
292 <term>The Linux kernel is portable</term>
293 <listitem>
294 <para>
295 Let's keep it that way. Your code should be 64-bit clean,
296 and endian-independent. You should also minimize CPU
297 specific stuff, e.g. inline assembly should be cleanly
298 encapsulated and minimized to ease porting. Generally it
299 should be restricted to the architecture-dependent part of
300 the kernel tree.
301 </para>
302 </listitem>
303 </varlistentry>
304 </variablelist>
305 </chapter>
306 307 <chapter id="ioctls">
308 <title>ioctls: Not writing a new system call</title>
309 310 <para>
311 A system call generally looks like this
312 </para>
313 314 <programlisting>
315asmlinkage long sys_mycall(int arg)
316{
317 return 0;
318}
319 </programlisting>
320 321 <para>
322 First, in most cases you don't want to create a new system call.
323 You create a character device and implement an appropriate ioctl
324 for it. This is much more flexible than system calls, doesn't have
325 to be entered in every architecture's
326 <filename class="headerfile">include/asm/unistd.h</filename> and
327 <filename>arch/kernel/entry.S</filename> file, and is much more
328 likely to be accepted by Linus.
329 </para>
330 331 <para>
332 If all your routine does is read or write some parameter, consider
333 implementing a <function>sysfs</function> interface instead.
334 </para>
335 336 <para>
337 Inside the ioctl you're in user context to a process. When a
338 error occurs you return a negated errno (see
339 <filename class="headerfile">include/linux/errno.h</filename>),
340 otherwise you return <returnvalue>0</returnvalue>.
341 </para>
342 343 <para>
344 After you slept you should check if a signal occurred: the
345 Unix/Linux way of handling signals is to temporarily exit the
346 system call with the <constant>-ERESTARTSYS</constant> error. The
347 system call entry code will switch back to user context, process
348 the signal handler and then your system call will be restarted
349 (unless the user disabled that). So you should be prepared to
350 process the restart, e.g. if you're in the middle of manipulating
351 some data structure.
352 </para>
353 354 <programlisting>
355if (signal_pending(current))
356 return -ERESTARTSYS;
357 </programlisting>
358 359 <para>
360 If you're doing longer computations: first think userspace. If you
361 <emphasis>really</emphasis> want to do it in kernel you should
362 regularly check if you need to give up the CPU (remember there is
363 cooperative multitasking per CPU). Idiom:
364 </para>
365 366 <programlisting>
367cond_resched(); /* Will sleep */
368 </programlisting>
369 370 <para>
371 A short note on interface design: the UNIX system call motto is
372 "Provide mechanism not policy".
373 </para>
374 </chapter>
375 376 <chapter id="deadlock-recipes">
377 <title>Recipes for Deadlock</title>
378 379 <para>
380 You cannot call any routines which may sleep, unless:
381 </para>
382 <itemizedlist>
383 <listitem>
384 <para>
385 You are in user context.
386 </para>
387 </listitem>
388 389 <listitem>
390 <para>
391 You do not own any spinlocks.
392 </para>
393 </listitem>
394 395 <listitem>
396 <para>
397 You have interrupts enabled (actually, Andi Kleen says
398 that the scheduling code will enable them for you, but
399 that's probably not what you wanted).
400 </para>
401 </listitem>
402 </itemizedlist>
403 404 <para>
405 Note that some functions may sleep implicitly: common ones are
406 the user space access functions (*_user) and memory allocation
407 functions without <symbol>GFP_ATOMIC</symbol>.
408 </para>
409 410 <para>
411 You should always compile your kernel
412 <symbol>CONFIG_DEBUG_ATOMIC_SLEEP</symbol> on, and it will warn
413 you if you break these rules. If you <emphasis>do</emphasis> break
414 the rules, you will eventually lock up your box.
415 </para>
416 417 <para>
418 Really.
419 </para>
420 </chapter>
421 422 <chapter id="common-routines">
423 <title>Common Routines</title>
424 425 <sect1 id="routines-printk">
426 <title>
427 <function>printk()</function>
428 <filename class="headerfile">include/linux/kernel.h</filename>
429 </title>
430 431 <para>
432 <function>printk()</function> feeds kernel messages to the
433 console, dmesg, and the syslog daemon. It is useful for debugging
434 and reporting errors, and can be used inside interrupt context,
435 but use with caution: a machine which has its console flooded with
436 printk messages is unusable. It uses a format string mostly
437 compatible with ANSI C printf, and C string concatenation to give
438 it a first "priority" argument:
439 </para>
440 441 <programlisting>
442printk(KERN_INFO "i = %u\n", i);
443 </programlisting>
444 445 <para>
446 See <filename class="headerfile">include/linux/kernel.h</filename>;
447 for other KERN_ values; these are interpreted by syslog as the
448 level. Special case: for printing an IP address use
449 </para>
450 451 <programlisting>
452__be32 ipaddress;
453printk(KERN_INFO "my ip: %pI4\n", &amp;ipaddress);
454 </programlisting>
455 456 <para>
457 <function>printk()</function> internally uses a 1K buffer and does
458 not catch overruns. Make sure that will be enough.
459 </para>
460 461 <note>
462 <para>
463 You will know when you are a real kernel hacker
464 when you start typoing printf as printk in your user programs :)
465 </para>
466 </note>
467 468 <!--- From the Lions book reader department -->
469 470 <note>
471 <para>
472 Another sidenote: the original Unix Version 6 sources had a
473 comment on top of its printf function: "Printf should not be
474 used for chit-chat". You should follow that advice.
475 </para>
476 </note>
477 </sect1>
478 479 <sect1 id="routines-copy">
480 <title>
481 <function>copy_[to/from]_user()</function>
482 /
483 <function>get_user()</function>
484 /
485 <function>put_user()</function>
486 <filename class="headerfile">include/asm/uaccess.h</filename>
487 </title>
488 489 <para>
490 <emphasis>[SLEEPS]</emphasis>
491 </para>
492 493 <para>
494 <function>put_user()</function> and <function>get_user()</function>
495 are used to get and put single values (such as an int, char, or
496 long) from and to userspace. A pointer into userspace should
497 never be simply dereferenced: data should be copied using these
498 routines. Both return <constant>-EFAULT</constant> or 0.
499 </para>
500 <para>
501 <function>copy_to_user()</function> and
502 <function>copy_from_user()</function> are more general: they copy
503 an arbitrary amount of data to and from userspace.
504 <caution>
505 <para>
506 Unlike <function>put_user()</function> and
507 <function>get_user()</function>, they return the amount of
508 uncopied data (ie. <returnvalue>0</returnvalue> still means
509 success).
510 </para>
511 </caution>
512 [Yes, this moronic interface makes me cringe. The flamewar comes up every year or so. --RR.]
513 </para>
514 <para>
515 The functions may sleep implicitly. This should never be called
516 outside user context (it makes no sense), with interrupts
517 disabled, or a spinlock held.
518 </para>
519 </sect1>
520 521 <sect1 id="routines-kmalloc">
522 <title><function>kmalloc()</function>/<function>kfree()</function>
523 <filename class="headerfile">include/linux/slab.h</filename></title>
524 525 <para>
526 <emphasis>[MAY SLEEP: SEE BELOW]</emphasis>
527 </para>
528 529 <para>
530 These routines are used to dynamically request pointer-aligned
531 chunks of memory, like malloc and free do in userspace, but
532 <function>kmalloc()</function> takes an extra flag word.
533 Important values:
534 </para>
535 536 <variablelist>
537 <varlistentry>
538 <term>
539 <constant>
540 GFP_KERNEL
541 </constant>
542 </term>
543 <listitem>
544 <para>
545 May sleep and swap to free memory. Only allowed in user
546 context, but is the most reliable way to allocate memory.
547 </para>
548 </listitem>
549 </varlistentry>
550 551 <varlistentry>
552 <term>
553 <constant>
554 GFP_ATOMIC
555 </constant>
556 </term>
557 <listitem>
558 <para>
559 Don't sleep. Less reliable than <constant>GFP_KERNEL</constant>,
560 but may be called from interrupt context. You should
561 <emphasis>really</emphasis> have a good out-of-memory
562 error-handling strategy.
563 </para>
564 </listitem>
565 </varlistentry>
566 567 <varlistentry>
568 <term>
569 <constant>
570 GFP_DMA
571 </constant>
572 </term>
573 <listitem>
574 <para>
575 Allocate ISA DMA lower than 16MB. If you don't know what that
576 is you don't need it. Very unreliable.
577 </para>
578 </listitem>
579 </varlistentry>
580 </variablelist>
581 582 <para>
583 If you see a <errorname>sleeping function called from invalid
584 context</errorname> warning message, then maybe you called a
585 sleeping allocation function from interrupt context without
586 <constant>GFP_ATOMIC</constant>. You should really fix that.
587 Run, don't walk.
588 </para>
589 590 <para>
591 If you are allocating at least <constant>PAGE_SIZE</constant>
592 (<filename class="headerfile">include/asm/page.h</filename>) bytes,
593 consider using <function>__get_free_pages()</function>
594 595 (<filename class="headerfile">include/linux/mm.h</filename>). It
596 takes an order argument (0 for page sized, 1 for double page, 2
597 for four pages etc.) and the same memory priority flag word as
598 above.
599 </para>
600 601 <para>
602 If you are allocating more than a page worth of bytes you can use
603 <function>vmalloc()</function>. It'll allocate virtual memory in
604 the kernel map. This block is not contiguous in physical memory,
605 but the <acronym>MMU</acronym> makes it look like it is for you
606 (so it'll only look contiguous to the CPUs, not to external device
607 drivers). If you really need large physically contiguous memory
608 for some weird device, you have a problem: it is poorly supported
609 in Linux because after some time memory fragmentation in a running
610 kernel makes it hard. The best way is to allocate the block early
611 in the boot process via the <function>alloc_bootmem()</function>
612 routine.
613 </para>
614 615 <para>
616 Before inventing your own cache of often-used objects consider
617 using a slab cache in
618 <filename class="headerfile">include/linux/slab.h</filename>
619 </para>
620 </sect1>
621 622 <sect1 id="routines-current">
623 <title><function>current</function>
624 <filename class="headerfile">include/asm/current.h</filename></title>
625 626 <para>
627 This global variable (really a macro) contains a pointer to
628 the current task structure, so is only valid in user context.
629 For example, when a process makes a system call, this will
630 point to the task structure of the calling process. It is
631 <emphasis>not NULL</emphasis> in interrupt context.
632 </para>
633 </sect1>
634 635 <sect1 id="routines-udelay">
636 <title><function>mdelay()</function>/<function>udelay()</function>
637 <filename class="headerfile">include/asm/delay.h</filename>
638 <filename class="headerfile">include/linux/delay.h</filename>
639 </title>
640 641 <para>
642 The <function>udelay()</function> and <function>ndelay()</function> functions can be used for small pauses.
643 Do not use large values with them as you risk
644 overflow - the helper function <function>mdelay()</function> is useful
645 here, or consider <function>msleep()</function>.
646 </para>
647 </sect1>
648 649 <sect1 id="routines-endian">
650 <title><function>cpu_to_be32()</function>/<function>be32_to_cpu()</function>/<function>cpu_to_le32()</function>/<function>le32_to_cpu()</function>
651 <filename class="headerfile">include/asm/byteorder.h</filename>
652 </title>
653 654 <para>
655 The <function>cpu_to_be32()</function> family (where the "32" can
656 be replaced by 64 or 16, and the "be" can be replaced by "le") are
657 the general way to do endian conversions in the kernel: they
658 return the converted value. All variations supply the reverse as
659 well: <function>be32_to_cpu()</function>, etc.
660 </para>
661 662 <para>
663 There are two major variations of these functions: the pointer
664 variation, such as <function>cpu_to_be32p()</function>, which take
665 a pointer to the given type, and return the converted value. The
666 other variation is the "in-situ" family, such as
667 <function>cpu_to_be32s()</function>, which convert value referred
668 to by the pointer, and return void.
669 </para>
670 </sect1>
671 672 <sect1 id="routines-local-irqs">
673 <title><function>local_irq_save()</function>/<function>local_irq_restore()</function>
674 <filename class="headerfile">include/asm/system.h</filename>
675 </title>
676 677 <para>
678 These routines disable hard interrupts on the local CPU, and
679 restore them. They are reentrant; saving the previous state in
680 their one <varname>unsigned long flags</varname> argument. If you
681 know that interrupts are enabled, you can simply use
682 <function>local_irq_disable()</function> and
683 <function>local_irq_enable()</function>.
684 </para>
685 </sect1>
686 687 <sect1 id="routines-softirqs">
688 <title><function>local_bh_disable()</function>/<function>local_bh_enable()</function>
689 <filename class="headerfile">include/linux/interrupt.h</filename></title>
690 691 <para>
692 These routines disable soft interrupts on the local CPU, and
693 restore them. They are reentrant; if soft interrupts were
694 disabled before, they will still be disabled after this pair
695 of functions has been called. They prevent softirqs and tasklets
696 from running on the current CPU.
697 </para>
698 </sect1>
699 700 <sect1 id="routines-processorids">
701 <title><function>smp_processor_id</function>()
702 <filename class="headerfile">include/asm/smp.h</filename></title>
703 704 <para>
705 <function>get_cpu()</function> disables preemption (so you won't
706 suddenly get moved to another CPU) and returns the current
707 processor number, between 0 and <symbol>NR_CPUS</symbol>. Note
708 that the CPU numbers are not necessarily continuous. You return
709 it again with <function>put_cpu()</function> when you are done.
710 </para>
711 <para>
712 If you know you cannot be preempted by another task (ie. you are
713 in interrupt context, or have preemption disabled) you can use
714 smp_processor_id().
715 </para>
716 </sect1>
717 718 <sect1 id="routines-init">
719 <title><type>__init</type>/<type>__exit</type>/<type>__initdata</type>
720 <filename class="headerfile">include/linux/init.h</filename></title>
721 722 <para>
723 After boot, the kernel frees up a special section; functions
724 marked with <type>__init</type> and data structures marked with
725 <type>__initdata</type> are dropped after boot is complete: similarly
726 modules discard this memory after initialization. <type>__exit</type>
727 is used to declare a function which is only required on exit: the
728 function will be dropped if this file is not compiled as a module.
729 See the header file for use. Note that it makes no sense for a function
730 marked with <type>__init</type> to be exported to modules with
731 <function>EXPORT_SYMBOL()</function> - this will break.
732 </para>
733 734 </sect1>
735 736 <sect1 id="routines-init-again">
737 <title><function>__initcall()</function>/<function>module_init()</function>
738 <filename class="headerfile">include/linux/init.h</filename></title>
739 <para>
740 Many parts of the kernel are well served as a module
741 (dynamically-loadable parts of the kernel). Using the
742 <function>module_init()</function> and
743 <function>module_exit()</function> macros it is easy to write code
744 without #ifdefs which can operate both as a module or built into
745 the kernel.
746 </para>
747 748 <para>
749 The <function>module_init()</function> macro defines which
750 function is to be called at module insertion time (if the file is
751 compiled as a module), or at boot time: if the file is not
752 compiled as a module the <function>module_init()</function> macro
753 becomes equivalent to <function>__initcall()</function>, which
754 through linker magic ensures that the function is called on boot.
755 </para>
756 757 <para>
758 The function can return a negative error number to cause
759 module loading to fail (unfortunately, this has no effect if
760 the module is compiled into the kernel). This function is
761 called in user context with interrupts enabled, so it can sleep.
762 </para>
763 </sect1>
764 765 <sect1 id="routines-moduleexit">
766 <title> <function>module_exit()</function>
767 <filename class="headerfile">include/linux/init.h</filename> </title>
768 769 <para>
770 This macro defines the function to be called at module removal
771 time (or never, in the case of the file compiled into the
772 kernel). It will only be called if the module usage count has
773 reached zero. This function can also sleep, but cannot fail:
774 everything must be cleaned up by the time it returns.
775 </para>
776 777 <para>
778 Note that this macro is optional: if it is not present, your
779 module will not be removable (except for 'rmmod -f').
780 </para>
781 </sect1>
782 783 <sect1 id="routines-module-use-counters">
784 <title> <function>try_module_get()</function>/<function>module_put()</function>
785 <filename class="headerfile">include/linux/module.h</filename></title>
786 787 <para>
788 These manipulate the module usage count, to protect against
789 removal (a module also can't be removed if another module uses one
790 of its exported symbols: see below). Before calling into module
791 code, you should call <function>try_module_get()</function> on
792 that module: if it fails, then the module is being removed and you
793 should act as if it wasn't there. Otherwise, you can safely enter
794 the module, and call <function>module_put()</function> when you're
795 finished.
796 </para>
797 798 <para>
799 Most registerable structures have an
800 <structfield>owner</structfield> field, such as in the
801 <structname>file_operations</structname> structure. Set this field
802 to the macro <symbol>THIS_MODULE</symbol>.
803 </para>
804 </sect1>
805 806 <!-- add info on new-style module refcounting here -->
807 </chapter>
808 809 <chapter id="queues">
810 <title>Wait Queues
811 <filename class="headerfile">include/linux/wait.h</filename>
812 </title>
813 <para>
814 <emphasis>[SLEEPS]</emphasis>
815 </para>
816 817 <para>
818 A wait queue is used to wait for someone to wake you up when a
819 certain condition is true. They must be used carefully to ensure
820 there is no race condition. You declare a
821 <type>wait_queue_head_t</type>, and then processes which want to
822 wait for that condition declare a <type>wait_queue_t</type>
823 referring to themselves, and place that in the queue.
824 </para>
825 826 <sect1 id="queue-declaring">
827 <title>Declaring</title>
828 829 <para>
830 You declare a <type>wait_queue_head_t</type> using the
831 <function>DECLARE_WAIT_QUEUE_HEAD()</function> macro, or using the
832 <function>init_waitqueue_head()</function> routine in your
833 initialization code.
834 </para>
835 </sect1>
836 837 <sect1 id="queue-waitqueue">
838 <title>Queuing</title>
839 840 <para>
841 Placing yourself in the waitqueue is fairly complex, because you
842 must put yourself in the queue before checking the condition.
843 There is a macro to do this:
844 <function>wait_event_interruptible()</function>
845 846 <filename class="headerfile">include/linux/wait.h</filename> The
847 first argument is the wait queue head, and the second is an
848 expression which is evaluated; the macro returns
849 <returnvalue>0</returnvalue> when this expression is true, or
850 <returnvalue>-ERESTARTSYS</returnvalue> if a signal is received.
851 The <function>wait_event()</function> version ignores signals.
852 </para>
853 <para>
854 Do not use the <function>sleep_on()</function> function family -
855 it is very easy to accidentally introduce races; almost certainly
856 one of the <function>wait_event()</function> family will do, or a
857 loop around <function>schedule_timeout()</function>. If you choose
858 to loop around <function>schedule_timeout()</function> remember
859 you must set the task state (with
860 <function>set_current_state()</function>) on each iteration to avoid
861 busy-looping.
862 </para>
863 864 </sect1>
865 866 <sect1 id="queue-waking">
867 <title>Waking Up Queued Tasks</title>
868 869 <para>
870 Call <function>wake_up()</function>
871 872 <filename class="headerfile">include/linux/wait.h</filename>;,
873 which will wake up every process in the queue. The exception is
874 if one has <constant>TASK_EXCLUSIVE</constant> set, in which case
875 the remainder of the queue will not be woken. There are other variants
876 of this basic function available in the same header.
877 </para>
878 </sect1>
879 </chapter>
880 881 <chapter id="atomic-ops">
882 <title>Atomic Operations</title>
883 884 <para>
885 Certain operations are guaranteed atomic on all platforms. The
886 first class of operations work on <type>atomic_t</type>
887 888 <filename class="headerfile">include/asm/atomic.h</filename>; this
889 contains a signed integer (at least 32 bits long), and you must use
890 these functions to manipulate or read atomic_t variables.
891 <function>atomic_read()</function> and
892 <function>atomic_set()</function> get and set the counter,
893 <function>atomic_add()</function>,
894 <function>atomic_sub()</function>,
895 <function>atomic_inc()</function>,
896 <function>atomic_dec()</function>, and
897 <function>atomic_dec_and_test()</function> (returns
898 <returnvalue>true</returnvalue> if it was decremented to zero).
899 </para>
900 901 <para>
902 Yes. It returns <returnvalue>true</returnvalue> (i.e. != 0) if the
903 atomic variable is zero.
904 </para>
905 906 <para>
907 Note that these functions are slower than normal arithmetic, and
908 so should not be used unnecessarily.
909 </para>
910 911 <para>
912 The second class of atomic operations is atomic bit operations on an
913 <type>unsigned long</type>, defined in
914 915 <filename class="headerfile">include/linux/bitops.h</filename>. These
916 operations generally take a pointer to the bit pattern, and a bit
917 number: 0 is the least significant bit.
918 <function>set_bit()</function>, <function>clear_bit()</function>
919 and <function>change_bit()</function> set, clear, and flip the
920 given bit. <function>test_and_set_bit()</function>,
921 <function>test_and_clear_bit()</function> and
922 <function>test_and_change_bit()</function> do the same thing,
923 except return true if the bit was previously set; these are
924 particularly useful for atomically setting flags.
925 </para>
926 927 <para>
928 It is possible to call these operations with bit indices greater
929 than BITS_PER_LONG. The resulting behavior is strange on big-endian
930 platforms though so it is a good idea not to do this.
931 </para>
932 </chapter>
933 934 <chapter id="symbols">
935 <title>Symbols</title>
936 937 <para>
938 Within the kernel proper, the normal linking rules apply
939 (ie. unless a symbol is declared to be file scope with the
940 <type>static</type> keyword, it can be used anywhere in the
941 kernel). However, for modules, a special exported symbol table is
942 kept which limits the entry points to the kernel proper. Modules
943 can also export symbols.
944 </para>
945 946 <sect1 id="sym-exportsymbols">
947 <title><function>EXPORT_SYMBOL()</function>
948 <filename class="headerfile">include/linux/module.h</filename></title>
949 950 <para>
951 This is the classic method of exporting a symbol: dynamically
952 loaded modules will be able to use the symbol as normal.
953 </para>
954 </sect1>
955 956 <sect1 id="sym-exportsymbols-gpl">
957 <title><function>EXPORT_SYMBOL_GPL()</function>
958 <filename class="headerfile">include/linux/module.h</filename></title>
959 960 <para>
961 Similar to <function>EXPORT_SYMBOL()</function> except that the
962 symbols exported by <function>EXPORT_SYMBOL_GPL()</function> can
963 only be seen by modules with a
964 <function>MODULE_LICENSE()</function> that specifies a GPL
965 compatible license. It implies that the function is considered
966 an internal implementation issue, and not really an interface.
967 </para>
968 </sect1>
969 </chapter>
970 971 <chapter id="conventions">
972 <title>Routines and Conventions</title>
973 974 <sect1 id="conventions-doublelinkedlist">
975 <title>Double-linked lists
976 <filename class="headerfile">include/linux/list.h</filename></title>
977 978 <para>
979 There used to be three sets of linked-list routines in the kernel
980 headers, but this one is the winner. If you don't have some
981 particular pressing need for a single list, it's a good choice.
982 </para>
983 984 <para>
985 In particular, <function>list_for_each_entry</function> is useful.
986 </para>
987 </sect1>
988 989 <sect1 id="convention-returns">
990 <title>Return Conventions</title>
991 992 <para>
993 For code called in user context, it's very common to defy C
994 convention, and return <returnvalue>0</returnvalue> for success,
995 and a negative error number
996 (eg. <returnvalue>-EFAULT</returnvalue>) for failure. This can be
997 unintuitive at first, but it's fairly widespread in the kernel.
998 </para>
9991000 <para>